1. Field of Invention
The present invention is directed to the disposal of organic wastes through composting and, more specifically, to composting systems for large scale treatment of industrial and municipal wastes.
2. Discussion of Related Art
Composting is a biological process of decomposition. Given adequate time and the proper environmental conditions, microorganisms turn raw organic matter into stabilized products. The products of composting include carbon dioxide, water, and a complex form of organic matter called compost, which is especially useful as a soil amendment. For the purposes of this specification “compost” shall be defined as “a mixture of biodried, biostabilized biosolids.” Parameters of importance in composting process management are the available carbon to nitrogen ratio, the moisture content, the oxygen content, and the temperature of the composting material.
Carbon serves primarily as a food source for the microorganisms involved in composting. Nitrogen is the primary constituent of protein which forms over 50% of dry bacterial cell mass and is, therefore, necessary for protein synthesis and the optimal growth of microbial populations in composting matter. It is well known in the art that the preferred carbon-to-nitrogen ratio for composting is about 20-40 parts carbon for each part nitrogen by weight (20:1-40:1). At lower ratios, the excess nitrogen supplied would be expected to be lost in the form of volatile nitrogen compounds, such as ammonia gas, and can cause undesirable odors or other environmental problems. Higher carbon-to-nitrogen ratios result in an insufficient supply of nitrogen for optimal microbial population growth, resulting in a slow rate of degradation.
The carbon-to-nitrogen ratio can be increased through the addition of materials high in carbon, such as fallen leaves, straw, woodchips, sawdust, bark, paper, cardboard, and the like. These types of materials will herein be referred to as “traditional compost amendment materials,” “amendment materials,” or “traditional amendment materials.” These types of materials are typically high in cellulose. For example, about 33 percent of most plant matter is cellulose (the cellulose content of cotton is 90 percent and that of wood is 50 percent.)
The carbon-to-nitrogen ratio can be decreased by the addition of materials high in nitrogen, such as vegetables, coffee grounds, grass clippings, manure, sewage, or the like. Sewage, such as that which is commonly supplied as influent into a typical municipal wastewater treatment plant, is not significantly cellulosic, containing little cellulose other than trace amounts due to undigested plant material, paper, or other such material present in the waste stream. As used herein, the term “non-cellulosic materials” will encompass materials which are substantially non-cellulosic, for example, sewage, and dewatered municipal wastewater sludge. Also, as used herein, the terms “sludge,” “sewage sludge,” “mixed sludge,” “municipal wastewater sludge” and the like will encompass both primary sludge (sludge which includes solids settled out, skimmed off the surface of influent wastewater in a primary clarifier, or otherwise mechanically separated from the wastewater prior to secondary treatment) as well as waste activated sludge (sludge which includes solids, including microorganisms, removed from a secondary treatment aeration basin or other secondary treatment process) and combinations thereof.
In some locations, the availability of materials such as wood chips or waste paper products, or other materials traditionally used as sources of carbon, or as bulking agents to add porosity to a feed mix for a composting operation, may be limited, or may be prohibitively expensive for use in composting operations. These types of materials may in some instances be preferably used for fuel for heating or energy production, for animal bedding, landscaping mulch, biofilter media, or for other purposes. The availability of these traditional compost amendment materials may vary by season. For example, demand for wood chips for use as a fuel for heating may increase in cold seasons in some locations. Sourcing, transportation, and storage expenses may also make the provision of cellulosic or other traditional compost amendment materials prohibitively expensive, especially if these materials are in short supply or not available proximate the composting operation.
Moisture content is another key parameter of composting material. Microbially induced decomposition occurs most rapidly in liquid films found on the surfaces of organic particles. Whereas inadequate moisture content inhibits bacterial activity, excess moisture content can inhibit the aerobic process. Excess water may plug openings or open space in the material inhibiting permeation and movement of air (oxygen) into and (carbon dioxide) out of the composting material, with the consequence of formation of anaerobic conditions. This anaerobic activity can produce undesirable odorous compounds, such as hydrogen sulfide or methane. The moisture content of a composting pile is typically related to the carbon-to-nitrogen ratio in that degradable materials that are high in carbon are typically correspondingly low in moisture; materials that are high in nitrogen are typically high in moisture.
Excessive moisture content can also result in the leaching of essential nutrients from the composting pile, including phosphorus, potassium, and other trace minerals, which are essential to microbial metabolism. Although these nutrients are not normally limiting, they must be present in adequate supplies for microbial activity.
Oxygen content and temperature are important environmental parameters of composting that fluctuate in response to microbial activity which consumes oxygen and generates heat. As microorganisms oxidize carbon for energy, oxygen is used up, and carbon dioxide is produced. Without sufficient oxygen the process will become anaerobic and produce odorous compounds such as volatile acids and reduced sulfur and nitrogen compounds. Oxygen content is also linked to moisture content in that excessive moisture content can reduce the available oxygen supply resulting in anaerobic pockets within the composting pile. Oxygen can be provided to microbes in a composting pile through the introduction of air into the pile, given that the pile has sufficient porosity to permit the air to permeate the pile.
The temperature of a composting pile varies according to the type and size of the microorganism community resident therein. Mesophilic microorganisms are dominant from the initial stage of decomposition until the temperature rises above about 40° C. and rapidly break down the soluble, readily degradable compounds. The mesophilic microorganisms become less competitive as the temperature rises above about 40° C., and thermophilic microorganisms take over.
As composting has become increasingly popular in recent years as a means for recycling a variety of organic materials as part of municipal and industrial solid waste management programs, various composting technologies have been or are being developed. These technologies include, for example, static pile composting, windrow composting, aerated windrow composting, and in-vessel composting employing horizontal agitated bay reactors and vertical reactors. In such systems, cost effectiveness and automation are typically desirable. Regarding cost, reducing the space required for a given throughput of composting material is a well recognized need in the industry. Composting operations employing windrows, for example, are thought to have an undesirably low ratio of composting materials throughput to processing area square footage. In in-vessel and closed reactors, compost material may typically be mounded up to 20 feet high. This mounding, however, produces technical difficulties regarding the adequacy of aeration in the reactor vessel leading, in some cases, to unacceptably large pockets of anaerobic activity within the pile. This anaerobic activity leads to the equally undesirable need for removal of odorous compounds from the reactor vessel environment before exhausting it to the atmosphere.
Certain in-vessel composting systems, particularly those comprising open bays within a building, have been used with excellent results. One system of this type, the IPS™ composting system, available from Siemens Water Technologies Corp. (Warrendale, Pa., USA), employs one or more automated agitators to thoroughly mix and aerate composting material in parallel bays. Starting at the discharge end of an open elongated composting bay, an agitator moves through the bed of composting material toward the loading end of the bay. Typically, the agitator travels through each bay mixing the material and rearwardly displacing it from the loading end of the bay toward the discharge end of the bay. In some agitator models, as the agitator progresses through the bay, a moveable member repeatedly repositions itself in the exhaust stream such that the distance of rearward displacement of composting material is gradually increased to accommodate material which has had progressively less residence time in the bay and, accordingly, has experienced less reduction in the volume due to decomposition and moisture content reduction. An agitator of this type is described by Hagen et al., in U.S. Pat. No. 5,387,036 which is incorporated herein in its entirety by reference.
As demands on municipal composting systems increase, the capability of treating larger volumes of composting material in relatively small reaction vessels becomes increasingly desirable. Accordingly, some existing systems having a plurality of open horizontal bays, typically between about 6 and 10 feet wide and up to about 300 feet long have been excessively loaded forming composting beds of increasing heights. A plurality of open bays are typically placed side by side and can be served by a single agitator. These systems can be used to compost a wide variety of materials, and the composting rate can typically be regulated to meet varying demand. When these large bays are at or near full capacity, however, the action of the rotating drum of the agitator tends to burrow through the composting pile as the agitator progresses through the bay. As the burrowing action continues, the weight of the undisplaced composting material can become sufficiently great to allow large quantities to suddenly collapse onto the rotating drum, thus slowing its rotation and, consequently, the agitator's progression through the bay. If a sufficient quantity of composting material collapses, the rotating mechanism can stall, resulting in costly delays while the unit is stopped, reversed, cleaned and/or repaired.
Furthermore, in a typical large scale composting operation, air contact along with the rising heat produced by microbial action may combine to dry the upper portion of the composting bed, forming a crust-like layer of composting material near the top of the composting bed. As the agitator travels through the bay, large sections of the crust remain intact for extended periods and then, suddenly, crack and fall onto the rotating drum, which increases the stalling frequency of the rotating mechanism.
In order to reduce the stalling frequency of the rotating mechanism of agitators used in some large scale composting operations, bridge breakers may be mounted to the agitator mechanism to assist in the breakup of the upper portion of the composting bed. These bridge breakers may comprise static or oscillating blades or other displacement mechanisms which may displace the composting material above the feeder, causing the material to fall substantially evenly onto the feeder, thereby preventing large amounts of composting material from suddenly collapsing onto the feeder which can result in overload conditions on the feeder drive mechanism. Bridge breakers of this type are described by Cole et al., in U.S. Pat. No. 5,906,436 which is incorporated herein in its entirety by reference.